The present invention relates to the measuring of the profile of a surface and in particular to measuring a profile using a confocal displacement sensor.
There are a multitude of applications where it is desirable to measure surface properties of a sample. The roughness of the surface, the curvature of the surface and the height of steps on the surface are typical application examples. Many diverse technologies can be employed to accomplish this metrology task. Common examples of devices include a contact profilometer, an atomic force microscope (AFM), a laser confocal displacement sensor, a laser triangulation displacement sensor, and a differential interferometer.
Each technology has advantages and disadvantages with respect to specific parameters, including vertical resolution, lateral resolution, maximum step size, and surface feature geometry. Other factors that determine the usefulness of a particular technology for a specific application include maximum measurement time, non-contact requirement, tilt correction, and material properties.
The principles associated with a confocal microscope as used in conventional laser confocal displacement sensors are described in U.S. Pat. No. 3,013,467, which is incorporated herein by reference. The basic principal of the confocal displacement sensor involves rejecting a large fraction of the light that is not in the focal plane of the surface of the sample, thereby increasing the contrast and resolution of the resultant image. One typical commercial version of a laser confocal displacement sensor is sold as the LT series by Keyence Corporation of America, located in Woodcliff Lake, N.J.
FIG. 1 is a block diagram of a conventional laser confocal displacement sensor 10 that is used to measure the surface of a sample 30. A laser 20 produces a beam of light that is focused by objective lens 32 onto the surface of sample 30. The beam strikes the surface of sample 30 at normal incidence.
After reflection from the surface of the sample 30 and transmission through objective lens 32, a fraction of the beam is reflected by beamsplitter 35 toward detector 60. Between detector 60 and beamsplitter 35 are a detector lens 40 and a pinhole 50. The focal plane of the objective lens 32 and the focal plane of the detector lens 40 (the plane of the pinhole 50) are made to be confocal. Typically, the objective lens 32 is scanned orthogonally with respect to the plane of the sample 30 over a range exceeding the expected step size on the surface of the sample using a piezoelectric or voice coil driver 36. An encoder associated with piezoelectric or voice coil driver 36 allows the determination of the position of the objective lens at any given time.
FIG. 2 shows a typical plot of the position of the objective lens along the X axis versus the measured detector signal intensity along the Y axis. The detector signal reaches a maximum when the beam is focused on the sample surface (and the pinhole) shown in FIG. 2 as position B. The detector signal falls off when the focal plane of the objective lens 32 is above or below the sample surface, shown as positions A and C in FIG. 2.
To make a measurement at one point on the surface of the sample 30, as shown in FIG. 1, the intensity measured by the detector 60 is recorded along with the position of the objective lens 32. The position of the lens 32 at the maximum detector intensity is determined and yields the relative height of the surface of the sample 30 at that measurement position. Two-point, line scans and area scans can be executed yielding the step height, line profile or area profile of a portion of the surface of the sample 30 by performing the measurement technique at a plurality of positions.
To obtain a more accurate and precise measurement using a standard laser confocal displacement sensor, the resultant height profile must be corrected for tilt. For example, if two points are measured, the tilt of the sample 30 can affect the height difference between the two points. To compensate for tilt, a plurality of positions located on a line through the two measurement points with the same height are measured, and the tilt of sample 30 with respect to the detector is calculated. The determined tilt is then used to correct (level) the measured height profile. The step height can then be calculated with improved accuracy and precision.
In addition, mechanical vibrations that cause a displacement of the sensor with respect to the sample surface will cause an error in the resultant height profile. Low frequency mechanical vibrations are common and can cause considerable measurement error. Minimizing this error source may require an expensive and specialized environment for the sensor. If the vibrations are originating from a process tool, isolation may be even more difficult. Another source of error is any vertical motion of the stage as it moves horizontally to translate the sample to another measurement location, especially if the measurement is made while the stage is moving. This will become a problem as the step height to be measured gets closer in magnitude to the vertical stage error.
An enhancement to the standard laser confocal displacement sensor is described in xe2x80x9cThe Optical Probe Using Differential Confocal Technique for Surface Profilexe2x80x9d by Wang, Fusheng, Tan, Jiubin and Zhao, Weiquan in Process Control and Inspection for Industry, Shulian, Wei Gao, Editors, Proceedings of SPIE vol. 4222 (2000), which is incorporated herein by reference. The enhanced laser confocal displacement sensor uses two unique sensors to receive the reflected signal. The light is reflected from a single point on the surface of the sample. The pinhole of the first detector is made to be a specific distance ahead of the confocal position while the pinhole of the second detector is made to be the same specific distance behind the confocal position. The difference between the two resultant detector intensity versus focal plane position curves yields a curve with a steep slope at the zero crossover point. The focal plane position at the zero crossover point indicates the height of the surface of the sample. The steeper the slope of the curve at this point, the better the resolution capability. As with the displacement sensor 10, shown in FIG. 1, the resultant height profile must be corrected for tilt and is subject to error caused by vibration.
What is needed is a displacement sensor that can accurately measure the surface profile of a sample without the need to correct for tilt and that is relatively insensitive to vibration errors.
A confocal displacement sensor in accordance with the present invention uses one or two laser wavelengths and produces two spots on a sample surface. The reflected intensities from the two spots are detected and measured after passing through one or more pinholes. Since the focal plane of the objective lens when focused on the sample surface and the focal plane of the detector lens (the plane of the pinhole) are confocal, the maximum detector intensity corresponds to the height of the surface for that point. This is done for both spots at each measurement location.
The resultant height profile advantageously does not need to be corrected for tilt as is common with all single point surface measurement techniques. A differential scan can be performed with the two spots relatively close together to generate the slope of the height profile. Integrating this profile yields the height profile of the scan. A referential scan can be performed by scanning the reference point across an area of constant height and the measurement point scanned across the feature to be measured to directly generate the height profile.
In accordance with one embodiment, the confocal displacement sensor includes at least one light source and a means for producing a first light beam and a second light beam. The displacement sensor also includes an objective lens for focusing the first light beam and the second light beam so that the beams are reflected by the surface of the sample. The displacement sensor includes a detector leg that receives the reflected first light beam and the reflected second light beam. The detector leg includes at least one detector lens and at least one detector and pinhole in the in the optical path of each reflected light beam. In one embodiment, two detectors and two pinholes are used. The at least one detector lens may have an opaque center portion.
A driver is coupled to one or more of the optical components to vary the focal planes of the one or more optical components so that it coincides with the surface of the sample. An encoder allows the precise measurement of the position of the one or more optical components. The position of maximum detector signal indicates the height of the surface at that location. For example, the driver may be coupled to the objective lens, both detector lenses or both pinholes.
A computer system is coupled to the detectors, the driver and the encoder and includes a computer-usable medium having computer-readable program code embodied therein for correlating and recording the intensities recorded by said first detector and said second detector over the range of positions of the driver and for calculating the height difference between the first spot and the second spot.
The means for producing a first light beam and a second light beam may include, e.g., two separate light sources that produce separate light beams that are made coincident by a beamsplitter or by fiber optic components. The coincident beams are then split using an optical component such as a Wollaston prism. The means for producing a first light beam and a second light beam may also be two light sources that produce two light beams that are not parallel. Alternatively, a single light source may produce a single light beam that is polarized and split by, e.g., a Wollaston prism.
A method of measuring the surface profile of a sample, includes producing a first light beam and a second light beam and focusing the light beams on the surface of the sample with an objective lens so that the first light beam and the second light beam are reflected off the surface of the sample. The reflected light beams are focused onto one or more detectors through one or more pinholes. A portion of the reflected light beams may be partially obscured prior to the reflected light being incident on the detectors. The focal plane of the objective lens is varied over a range, wherein the objective lens is confocal with the first pinhole and the second pinhole within that range. The relative height associated with the maximum intensity of each detector is determined. The maximum intensity represents the height of the surface of the sample where the first light beam and the second light beam are incident on the surface.
A first height difference between where the first light beam and the second light beam are incident on the surface is then determined based on the maximum intensities detected by the detectors. The sample can then be repositioned and the method repeated for the new position until a second height difference is calculated. The difference between the first height difference the second height difference can then be calculated to determine the surface profile of the sample. This difference is insensitive to any tilt of the sample with respect to the measurement tool and is relatively insensitive to vibrations.